1. Field of the Invention
The present invention relates to electroacoustic transducers whose vibrating element is formed of a piezoelectric material layer membrane having electrodes and being held at its periphery in a capsule case.
2. Description of the Prior Art
This type of capsule has been known for many years and is described for example in the documents GB-A-No. 2 046 554, JP-A-No. 58 202699, JP-A-No. 60 199298 or JP-A-No. 61 29298.
In these known capsules, the membrane is formed of a thin disk to which is fixed a piezoelectric ceramic layer covering the central zone of the disk, the two faces of the piezoelectric ceramic layer being metallized so as to form respectively an internal electrode applied against the thin disk and an external electrode. The membrane is housed in a case formed of a base body and a lid fitting one on the other along a closed generally circular periphery. The base body forms a first wall substantially parallel to the disk and defining with the disk a rear acoustic cavity. The lid forms a second wall substantially parallel to the disk and defining with the disk a front acoustic cavity. The case comprises a supporting surface, for example in the form of an inner peripheral step, against which the periphery of the disk is held applied by holding means. Electric connection means provide the respective electric connections of one and other of the electrodes, with a first and second external connection terminal of the case.
In known capsules, mechanical holding of the membrane in position with respect to the case and the electric connection of the electrodes and of the connection terminals are provided by separate means.
Thus, in the document GB-A-No. 2 046 554, the periphery of the disk bears on a peripheral supporting surface of the base body, against which it is held by a peripheral O-seal inserted between the lid and the disk; the electric connection is provided by conducting strips fixed to the disk and connecting the respective electrodes to a peripheral zone of the disk projecting from the case.
In the document JP-A-No. 58 202699, the disk is also held against a peripheral supporting surface of the base body, against which it is urged by a peripheral 0-seal inserted between the disk and the lid; the electric connections are provided on one hand by an electric conductor in the form of a wire soldered to an electric connection terminal and to the external electrode, and on the other hand by the disk itself which is made from an electrically conducting material in contact with the case which is also electrically conducting by its periphery.
In the document JP-A-No. 60 199298, the periphery of the disk bears against a peripheral supporting surface of the lid against which it is held by a peripheral annular seal inserted between the base body and the disk; one of the electric connections is provided by a resilient tongue, a first end of which is held against the base body and the other end of which bears on the piezoelectric element of the disk.
In the document JP-A-No. 61 29298, the periphery of the disk is embedded between the peripheries of the lid and the base body; one of the electric connections is provided by a resilient tongue a first end of which is fixed by a rivet to the centre of the base body and the other end of which bears on the piezoelectric element of the disk.
Known capsules are then relatively complex and require the provision of first means for mechanically holding the membrane in the case and second means providing the electric connection of the electrodes, and all these means must be assembled during assembly of the capsule.
The present invention has particularly as object to avoid the drawbacks of known capsules, by attempting to simplify the assembly and reduce the number of parts to be made and assembled, without adversely affecting the acoustic results obtained, and improving if possible these results.
According to another object of the invention, the new capsule structure holds the membrane in the case without using an O-seal; the inventors have in fact discovered that the presence of such O-seals in a capsule has drawbacks during large-scale production; in fact, known seals usable for large scale production are generally of dimensions which are not very regular, which induces defects in the evenness of the surfaces holding the piezoelectric membrane and variations in the acoustic qualities of the capsules thus obtained. The defects related to the presence of O-seals also result from the variation of thickness of the other stacked parts forming the capsule, particularly the base body and the lid, creating a nipping force on the periphery of the disk which is difficult to control. The mechanical holding means of the invention avoid such variations.
According to another object of the invention, the new capsule structure substantially increases the possibilities of deformation of the peripheral zone of the disk, so that the electroacoustic qualities of the capsule are substantially increased.
The invention further provides particular embodiments leading to the general improvement of the electroacoustic qualities of the capsule. In particular, attempts have been made to distribute the natural mechanical vibratory frequencies of the different elements of the capsule, so as to obtain the flattest possible response curve in the usual operating frequency range.
Another effect sought is to obtain a good compromise between the acoustic effect of the capsule and its size, particularly its thickness.